Electronic control system for motors and the like

ABSTRACT

A controller for a motor or like electrical equipment. A thyristor in series with a power conductor that feeds the equipment and a control circuit for activating the gate terminal of the thyristor in accordance with the operating conditions of the equipment. The control circuit includes a transformer having a secondary winding connected to the gate terminal and means for varying the current flow in the secondary winding according to load demands. A three phase system having a thyristor in each conductor and a transformer secondary winding for each thyristor, which windings are on a common core.

This is a division of Ser. No. 492,491, filed July 29, 1974 and nowabandoned and a continuation of Ser. No. 357,863, filed May 7, 1973 andnow abandoned.

BRIEF SUMMARY OF THE INVENTION

This invention relates to electronic means of controlling functions ofan electric motor or other electric load.

According to the present invention there is a control transformer thathas a core, a primary winding on the core, and a secondary winding onthe core. The secondary winding is connected to the gate terminal of athyristor which is connected in series with one of the power conductorsto a load, such as a motor. The current induced in the secondarywindings controls the thyristor which in turn controls the power to theload.

For controlling the magnitude and timing of current induced in thesecondary winding, the present invention provides several expedients. Ineach, a dropping resistor in series with the primary winding is employedso that the voltage across the primary winding increases as the currentthrough the series combination decreases. One expedient for effectingchange in secondary current is embodied in an auxiliary winding on thecore and circuitry for controlling current flow through the auxiliarywinding to alter the degree of saturation in the core. Altering thedegree of saturation in the core alters the load on the primary which inturn alters the current flow through the primary, the voltage across theprimary, and finally, the voltage induced across the secondary winding.

Another expedient for controlling the magnitude and timing of currentinduced in the secondary is the employment of a split core and one ormore ferrous bodies that are moveable between two or more positions toeffect variation in the degree of flux linkage between a primary portionof the split core and a secondary portion of the split core.

For controlling the position of the ferrous body or the current flowthrough the auxiliary winding, the invention includes control circuitrythat responds to load and/or speed conditions dictated by the nature ofthe load and the ratings of the equipment (e.g., a motor) to which poweris supplied through the system of the invention. Exemplary of thefunctions performed by the control circuitry are regulation of currentto the load during start, during initial warm-up, during load changes,and during overload. The control circuitry also affords reversal ofpolarity or phase relation, such as is useful in reversing the directionof rotation of electric motors. Since the control functions areperformed electronically at relatively low power levels, the inventioncan be accommodated to virtually any load in any environment.

It is an object of this invention to provide a solid state startingcontrol for an induction motor with a controlled starting time andregulated maximum current.

One object of this invention is to provide a solid state motorcontroller with a manual contact less reversible function, withcontrolled current regulation, and instant current overload protection.

One more object of this invention is to provide for a solid state motorprotection system controllable to fit the desired protection level ofeach motor.

A further object of this invention is to provide several separatefunctions in an overload control.

At the moment a motor is started, a higher current is required to forcea motor in motion that when a motor's speed is maintained in motion.Therefore, this invention provides for means of introducing apredetermined starting sensitivity value rated to the maximum startingcurrent of the motor, and to the normal starting time.

A separate adjustable means is then provided to monitor the normal loadcurrent of a motor with a time delayed means to allow for normal currentsurges from peak loads on the motor.

One more object of this invention is to provide for a temperaturecorrective device regulating the sensitivity of the overload control.

A machine when started up cold will consume a higher current untilheated up by friction within the mechanical components.

To compensate for this added load, the temperature corrective devicewill cause the overload control circuit to monitor the motor currentwith a slightly decreased sensitivity during the first few minutes ofoperation. However, a separate heat source will heat up the temperaturesensitive device after a predetermined length of time close to the timerequired for the machine to heat up. As the temperature sensitive deviceheats up, the internal resistance of the device is lowered to increasethe sensitivity of the overload control.

Another object of the overload control circuit is to provide a rapiddischarge of the capacitor which is a useful feature when the currentpulses from the motor are not high enough in amplitude or in duration totrigger the tripping device.

As soon as the current charging the capacitor is lowered to apredetermined minimum level, a voltage sensitive network functions todischarge the previously charged voltage in the capacitor.

One more specific object of this invention is to provide an electroniccircuit controlling the current in the secondary rotor windings of aninduction motor. The circuit components may be placed in an epoxy moldand fastened to the end of the rotor shaft where the three currentswitching semiconductor components may be inserted in a cast aluminumrotor cooling fan which will function as a most efficient heat sink.

A still more specific object of the invention is to provide anadjustable constant speed control performing unaffected by normal loadconditions.

One further object of this invention provides for a self-regulatedcurrent control circuit which is adjustable to enable a motor to operateat a regulated maximum current, and with a constant torque at allspeeds.

Accordingly, the primary object of this invention is to provide allmeans desirable in the controlling of an AC motor at a far greatersimplicity than previously possible. All the functional adjustmentsthrough the magnetic, resistive or voltage-sensitive means in theaforementioned adjustments may be assumed to be constructed withexternal resistive controls. Each control requires one slip ring withone conductor and resistance variable against ground potential.

One more objective is added to this invention by providing the overloadfunctions in the aforementioned circuits as overload control means forAC and DC motor control circuits.

Other objects and advantages of this invention will become apparent froma review of the drawings and the following specification.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a solid state motor controllerof a three-phase induction motor showing an electronically variabletransformer. An electronic start and stop circuit controlling thevariable transformer with controlled starting time. An instant currenttrip circuit. An adjustable constant torque circuit. A capacitordischarge circuit. An adjustable starting sensitivity control. Anelectronic overload control with temperature compensated sensitivitycontrolling the start and stop circuit.

FIG. 2 is a fragmentary view of an electronic motor controller similarto FIG. 1. A selective variable transformer is shown, operable byelectronic, mechanical and magnetic means.

FIG. 3 is a schematic view showing an electronic rotor control circuithaving several adjustable means controlling a multitude of functions ofa rotor. A variable inductive means provides a constant speed with anexternal speed control.

FIG. 4 represents a fractional view of an electronic rotor controlcircuit similar to FIG. 3 having a different thyristor control means,and a resistive constant speed control.

FIG. 5 represents a fractional view similar to FIG. 4, showing adifferent magnetic-inductive control means of the electronic rotorcontrol circuit.

FIG. 6 is a sine wave schematic diagram showing the phaseshift methodused in operating the aforementioned motor current controllingthyristors of FIGS. 1, 2, 3 and 9.

FIG. 7 is a schematic representation of a motor control overload circuitsimilar to FIG. 1. The control circuit is modified to control thecurrent feeding a DC motor, and to deactivate the motor control circuitat an overload condition.

FIG. 8 is a schematic showing a basic function of an overload control.

FIG. 9 illustrates a fragmentary view of the AC motor controllerssimilar to as shown in FIG. 1 modified to control the function of a DCmotor.

FIG. 10 is a fragmentary illustration showing an AC relay contactor witha manually operated solid state control circuit, and overload tripcontrolled by a circuit similar to circuits as shown in FIG. 1 and FIG.7.

FIG. 11 is a simplified fragmentary illustration similar to FIG. 10,overload trip controlled by a circuit similar to as shown in FIG. 1.

FIG. 12 is a fragmentary view showing a selective transformer having avariable magnetic control means.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following detailed description of the preferred embodiments ofthe present invention, similar reference numerals are used to designateelements in the various views that are structurally or functionallysimilar, unless the context clearly indicates otherwise. The last twodigits in the reference numerals are common insofar as the elements havestructural or functional similarity. For example, in FIG. 1 there is atransformer 115 that is functionally similar to transformer 215 in FIG.2, transformer 415 in FIG. 4 and transformer 1215 in FIG. 12.

Referring to FIG. 1, a three phase electronic motor starter isaccomplished by employment in each phase of a controlled bidirectionalthyristor 116, known as "triac," to control the AC motor current in eachphase. For simplicity, it is to be assumed the thyristor contains the"Diac" thyristor diode as a voltage regulating means built into its gatecircuit. The thyristors 116A, B and C are controlled through full wavephase shifting of the AC sine wave at the control gate of each thyristorby the function of the electronically variable transformer 115. As thethree phase voltage appears at motor feed conductors 118A, B and C, theprimary windings 186A, B and C of transformer 115 are energized throughrespective voltage controlling resistors 185A, B and C. The AC voltageinduced in the secondary-delta connected windings 188A, B and C isrectified full wave through diodes 113, and transistor 108 is switchedon saturated by a negative voltage on conductor 125 through resistor107. The resulting load on transformer 115 produces a voltage dropacross primary series resistors 185, and the voltage induced by thesecondary windings 187A, B and C does not reach sufficient amplitude oncapacitors 117A, B and C to turn on respective thyristors 116 throughresistors 161A, B and C. To start motor 101, push button 110 isdepressed, and the positive voltage on conductor 122 through resistor104 switches silicon controlled rectifier 120 known as "thyristor on."The positive voltage on conductor 122 through the normally closed stopbutton 100 and conductor 123 through SCR 120 appears at conductor 31charging capacitor 147. The negative side of capacitor 147 is brieflycharged from negative conductor 125 through resistor 145. However, ascapacitor 147 is fully charged, the stored voltage is discharged acrossvariable starting time resistor 144 at a time rate predetermined by theset value of said resistor.

As capacitor 147 is discharged, a positive voltage builds up rapidly onconductor 126. As the rising voltage on conductor 126 approaches themaximum voltage, the rate of the rising voltage is slower. At thatpredetermined voltage level, zener diode 128 conducts, and conductor125', connected to the base of transistor 108 changes from negative to amore positive voltage which functions to turn off transistor 108, and inturn to desaturate the core of transformer 115. This decreases thevoltage drop across the primary voltage regulating resistors 185, andthe rising voltage on the three phase star connected primary windings186 induces a rising voltage in the secondary coils 187 which isrectified full wave through diodes 132 thereby providing thyristors 116with a negative gate current and voltage for the positive as well as forthe negative switching cycle of the thyristors to comply with themanufacturer's recommended operation of the thyristors. Also tocompensate for the phase shift between line current and line voltagewhich appears during inductive loads, the dv/dt is limited by amanufacturer's RC circuit 173 connected in parallel with thyristors 116.

As the negative voltage charging capacitor 117 reaches the triggeringvoltage level of thyristor 116, the thyristor conducts at a maximumphase shifted sine wave with a time delay predetermined by resistors 161and the instantaneous voltage induced in secondary windings 187, whichis predetermined by the inductive reactance in the gradually unsaturatedmagnetic core of transformer 115 and primary voltage regulatingresistors 185.

As the conduction across the emitter-collector region of transistor 108decreases, transformer 115 offers less resistance, and capacitors 117trigger thyristors 116 with a less phase shifted voltage until theentire sine wave is conducted to motor 101.

Assuming induction motor 101 is started in this manner, the rapidlyrising voltage across the three phase motor terminals will start therotation of the motor rotor at a voltage value predetermined by the loadconditions on the motor. At the first movement of the rotor, a counterelectro-motive force is generated in the stator windings of motor 101serving to reduce the motor starting current. The rapidly increasingspeed of the rotor further increases the counter electro-motive force,and a lower starting current is obtained amounting to a fraction of thecurrent consumed when a motor is started with a regular motor contactor.Since induction motor 101 may be started at a pre-adjusted time rate byadjusting resistor 144, an invaluable function is obtained to save thewear and tear of mechanical parts.

The FIG. 1 overload control is accomplished by connecting currenttransformers 102 to each phase. The current picked up by thetransformers is conducted via conductors 190 to rectifiers 103 whichcooperate with other curcuit elements to control a transistor 52.

Transistor 52 serves a similar function to the manual push buttoncontact 810 in FIG. 8. In FIG. 8 when the motor 801 is started bydepressing start button 811, the contact is opened momentarilyintroducing the variable resistor 804 into the motor overload controlcircuit. The predetermined value of resistor 804 is rated to allow forthe normally high starting current of motor 801. However, motor starterbutton 811 must be depressed long enough for the motor starting currentto decrease to the running current value, which is normally less thanone second. The electronic circuit in FIG. 1 is arranged to maintaintransistor 52 in a normally saturated state thereby shorting outstarting sensitivity resistor 4. Two voltage reference diodes 154 areconnected in series to provide a regulated voltage sufficient toenergize transistor 52 and enables the transistor to remain energizedduring the lowest operating motor current. The remaining voltage appearsas a voltage drop across a variable resistor 55, and a current limitingresistor 50. When motor 101 is started, a rectified DC voltage appearsbetween conductors 31 and 25. A capacitor 56 instantly attracts theopposite polarity, causing a current flow through resistor 57 whichprovides a negative voltage to keep transistor 52 turned off for apredetermined length of time. This function introduces the pre-setresistance value of the variable resistor 4 into the overload controlcircuit resulting in a decreased sensitivity to allow for the normallyhigher starting current of motor 101. Capacitor 56 will discharge at atime rate predetermined by the set value of the variable resistor 55 andsince the variable resistor 55 is of a lower value than resistors 57,the voltage at conductor 58 will change to positive. As a result,transistor 52 becomes saturated, and resistor 4 is shunted out of thecircuit.

When the motor 101 reaches full speed, the sensitivity of the overloadcontrol is determined by the set value of resistor 5 which is rated tothe maximum load of the motor, or to any desired lesser load level. Thesensitivity is also automatically adjusted by the function of thermistor43. When a motor driving a machine (not illustrated) is started up cold,the current required is usually higher during the first minutes whileits mechanical parts are being warmed up by internal friction. As themotor is started, the heating element 45 is energized by power fromconductors 31 and 125. A heat resistive body 44 is provided as a meansto delay heating of thermistor 43 so that as the motor heats upthermistor 43 is correspondingly heated, and the resistance thereofdecreases. In order to achieve more stable performance of the overloadcontrol, a zener voltage reference diode 47 in series with a resistor 48is provided to function as a nonlinear resistor. As the voltage risesabove the rated reference voltage of diode 47, the diode will conduct tomaintain the same voltage level. Therefore a voltage rising at an evenrate will reduce to a slower rise time as a result of the increasingload of the nonlinear resistance of resistor 48 and zener diode 47.

Since only a small voltage drop is desired across resistor 48, a lowresistance value is preferred. If the voltage across capacitor 6 risesto the predetermined trip level of the four layer thyristor diode 130,the diode will fire instantly to discharge capacitor 6. Such dischargecauses a reverse polarity across SCR 120, and the SCR turns off,permitting a negative voltage across resistor 107 to energize transistor108. The instantaneous load on transformer windings 188 increases thevoltage drop across primary load resistors 185, and the induced voltagein secondary windings 187 drops below the triggering level ofbidirectional thyristors 116.

If the voltage in the aforementioned circuit of capacitor 6 fails toreach the rated trip value of thyristor diode 130 and the motor currentdecreases to a lower level, an electronic discharge circuit forcapacitor 6 functions instantly to discharge the capacitor to a lowervoltage level corresponding to the operating level of motor 101. Thisprovides a maximum RC time constant to function in a motor controlcircuit where the motor current appears with current surges of shortduration in rapid succession.

The electronic discharge circuit comprising resistor 32 develops avoltage drop that produces a negative bias to keep the electronicdischarge circuit turned off. As the motor current decreases, thenegative voltage at conductor 61 will appear more positive in respect tothe negative voltage stored in capacitor 6 and appearing on conductor29. A positive voltage from conductor 31 across resistor 67 will turn onthe direct coupled amplifier comprising a transistor 68 which turns ontransistor 69 to discharge capacitor 6 through peak current loadresistor 70. As capacitor 6 is discharged to a level where the voltageat conductor 61 is more negative than the remaining capacitor voltage,the resulting negative voltage on conductor 61 through resistor 64 willturn off transistors 68 and 69. The required voltage drop acrossresistor 32 to control the electronic discharge circuit is predeterminedby transistor 68 biasing network comprising resistors 64 and 67. Diodes65 function to protect the emitter-base junction of transistor 68against reverse voltage breakdown.

The electronic overload circuit represented as part of the electronicmotor control unit in FIG. 1 provides a means for instantaneous currentprotection of the bidirectional thyristors 116. Before the current inconductors 118 and 121 rises to the maximum rated value of thyristors116, the aforementioned circuit comprising variable resistor 137 chargescapacitor 139 to the voltage level where the four layer thyristor diode141 will fire to turn off the latched SCR 120 in a similar manner asthyristor diode 130. Resistor 138 functions as a discharge path forcapacitor 139 and enables variable resistor 137 to provide adjustment tothe aforementioned instant current control.

One more function is added to the FIG. 1 overload control enabling motor101 to operate at a constant maximum torque at any adjusted currentlevel. Potentiometer 150 can be adjusted to operate motor 101 with aregulated torque and instantaneous current control by regulatingpotentiometer 149 to zero time delay, or if desired, the time delaypotentiometer may be set to permit a higher current to handle loadsurges. In case of a rising current in motor conductors 118-121, anegative voltage appears on conductor 62. Capacitor 148 attracts apositive voltage from conductor 31 through potentiometer 150, and causesa time delay to prevent an instantaneous regulation of motor currentpeak loads. However, as soon as capacitor 148 has discharged across thepredetermined resistance-value set on potentiometer 149, a more negativevoltage in respect to the negative base conductor 126 appears onconductor 151, and as soon as the negative voltage rises to the ratedconduction level of zener diode 127, the diode conducts to regulatetransistor 108 to a conduction level where the motor current is reducedbelow the feed back voltage value of zener diode 127. The function ofblocking diode 124 is to prevent a rising positive voltage from chargingcapacitor 148 from regulating transistor 108.

FIG. 2 is a partial illustration representing a modified version of theFIG. 1 electronic motor control.

An electronically and mechanically variable transformer is incorporatedin the thyristor control circuit to provide a manual operation of theforward and reverse direction of the motor rotor, and a manual speedcontrol. The electronically variable means of transformer 115 in FIG. 1is also provided in the variable transformer 215 of FIG. 2, andcontrolled through conductors 259 by an electronic overload controlsimilar to that connected to conductors 159 in FIG. 1, and therefore notillustrated.

When motor 201 is operated by the aforementioned manual control,starting sensitivity resistor 4 of FIG. 1 and the controlling circuitcan be omitted, or adjusted to a low resistance value. Also theelectronic circuit controlling capacitor 6 and thyristor diode 130 canbe omitted, and motor 201 can be controlled by current regulationthrough the function of potentiometer 150. If a zero time delay is setby the function of potentiometer 149 and capacitor 148, motor 201 can bestarted manually with variable transformer control arm 264 with aninstantaneous current regulation, and transformer control arm 264 can beswitched directly from full speed forward to full speed reverse with thepredetermined torque value providing braking power and time rate forreversing speed. As motor 201 is started by a starting control circuitsimilar to that activated by starter button 110 in FIG. 1, theaforementioned function de-energizing transistor 108 serves to decreasegradually the DC current rectified by diodes 113 from the three phaseconductors 159 which correspond to conductors 259 in FIG. 2. As the ACcurrent through transformer windings 228 decreases, the voltage dropacross the voltage regulating resistors 285 decreases, and the risingvoltage across primary windings 286 induces a rising voltage in thesecondary windings 255A, B and C. The rising AC voltage induced inwinding 255A is rectified full-wave through diodes 235A, and throughconductor 256A and phase shifting resistor 260A charges capacitor 257Awith a voltage which initially represents a maximum phase shiftedvoltage to trigger thyristor 219A into conduction serving to switchmotor feed conductor 218A to motor conductor 221B. Meanwhile, therectified voltage induced in the windings 255B functions to triggerthyristors 219B switching motor feed conductor 218B to motor conductor221A. This mode of operation completes the phase reversing actioninduced by transformer windings 255A and B. The electromagnetic fluxgenerated by primary windings 286 flows through the sections of the ironcore associated with secondary transformer windings 255A, B and C; theelectromagnetic flux is diverted to bypass the sections of transformeriron cores associated with coils 287A and 287B. In addition, theelectromagnetic flux that is diverted represents the same direction offlow, and therefore functions to affect the degree of saturation of themagnetic core. To change the direction of motor rotation, the manualcontrol handle 264, pivoting on stationary shaft 268, is switched fromposition F to position R. This moves control arm 263, pivoting onstationary shaft 266, in an upward motion by the sliding motion ofprotruding shaft 265 in a non-tangent elongated slot 267 of controlhandle 264. The resulting upward motion of control rod 233 caused by theprotruding pin 270 sliding in the elongated slot 269 functions to movethe ferrous cores 234 to the upper seated position. The lower protrudingsection of ferrous core 234A functions to bridge the upper air gapbetween the primary and the secondary side of transformer 215. Theferrous core 234B moves up to bypass the magnetic flux flowing in thesection of the core that extends through coil 255A, while the lowerprotruding section of core 234B bridges the air gap between the primaryand secondary side of pole pieces located between primary phase windingsA and B. Ferrous core 234C moves up to bypass the magnetic flux flowingin the section of the core that extends through coil 255B, while itslower protruding section bridges the air gap of the pole pieces locatedbetween primary phase windings B and C. The ferrous core 234D moves upto a position at which the lower protruding section replaces the uppersection thereby leaving secondary coil 255 unaffected. As the ferrouscores are moved from their respective positions, all secondary coils 255and 287 become de-energized from lack of magnetic flux causing allthyristors to remain de-energized until the ferrous cores are seated intheir new positions. However, the phase shifting of the thyristors maybe controlled by moving ferrous cores 234 between their extremepositions. A remote control means is provided for the forward andreverse operation of transformer 215 by the function of electro-magnet227 acting on ferrous core 228 mounted on control rod 233. Manualcontrol arm 263 can be removed from the protruding pin 270, and magnet227 operated from an external power source through conductors 225 byswitch 226.

FIG. 3 shows a bidirectional thyristor controlled circuit similar toFIG. 1 used to control the functions of a rotor-wound induction motor.The rotor control circuits shown in FIGS. 3-4 and 5 are designed toprovide several new features to the performance and function of themotor, and greatly improve the economy of using a slip ring inductionmotor by eliminating the wasted power in the present high current loadresistors used for rotor speed control. In the design of the electronicrotor control circuit, the anode of the aforementioned thyristors aredesigned with common potential, and may be pressfitted into the body ofthe cast aluminum cooling fan of the rotor to function as a mostpractical and efficient heat sink at ground potential. The controllingtransformers are of miniature size, and the entire circuit can be placedin a circular mold filled with epoxy resin, and mounted next to thethyristors around the rotor shaft. The primary voltage is assumed to besupplied to motor 301 by the primary motor conductors 121 of FIG. 1motor controller, and marked by a phantom line in FIG. 3.

The primary voltage through voltage regulating resistors 385 is fedthrough conductors 320 and rotor slip rings 391 to the primary windings386 of variable transformer 315 controlling thyristors 316 in a similarmanner as transformer 115 controls thyristors 116 in FIG. 1. A variableinductive means is provided to regulate the rotor speed to a constantrate. Such means includes a variable speed control provided by thecoaction of ferrous core 378 controlled by a variable magnetic force ofcoil 312. Current through coil 312 is controlled from the groundedarmature potential through conductor 376, slip ring 352 and externalspeed controlling potentiometer 354 to a positive voltage on a conductor381, in respect to the negative grounded terminal 383.

The biasing network of transistor 308 is constituted by resistors 307and 311 in such a manner that a sufficient amount of voltage is inducedin secondary transformer windings 387 to trigger thyristors 316 into alow conduction, thereby permitting a voltage to be induced in the deltaconnected windings of current transformers 302 which voltage is ofsufficient amplitude to reach the regulated voltage level of zener diode336. The regulated voltage through resistor 349 and conductor 306charges capacitor 347 across the adjustable starting time ratepotentiometer 344. A negative voltage from conductor 331, resistor 335and conductor 310 is attracted to capacitor 347, so as to afford a timedelay until capacitor 347 discharges at a time rate set by potentiometer344. As the rapidly rising positive voltage increases, a slower rate ofrise is achieved, and at that critical point, zener diode 328 conductsto decrease the conduction of transistor 308. The resulting decreasingload of transformer 315 results in an increased conduction through saidtransformer, and a less phase shifted voltage triggers thyristors 316into a higher conduction rate. As the rotor speed increases, thecentrifugal force acts on control rod 333 and ferrous cores 334 and 378in the direction of arrow 342 until the centrifugal force is equal tothe predetermined force of magnet coil 312, and the rotor speed ispredetermined by the distance of removal of the ferrous cores 334 fromthe center of transformer coils 386 and 387. A resulting constant speedis achieved by adjusting potentiometer 354.

An instantaneous current control similar to FIG. 1 is provided, as isthe regulating current feature shown in FIG. 1 to regulate theconduction of transistor 308. External current control is providedthrough potentiometer 350 with an adjustable time delay being affordedby potentiometer 349 and capacitor 348. Voltage divider networkresistors 349 and 350 have a lower series resistance than resistor 351and function to produce a normally negative voltage on conductor 314. Asa high rotor current produces a high voltage between conductor 309 andground potential, the negative side of capacitor 348 is charged throughpotentiometer 350. A positive voltage through resistor 351 charges thepositive side of capacitor 348 to produce a delay. As soon as capacitor348 discharges, the negative voltage through potentiometers 350 and 349increases on conductor 314 to the rated conduction level of zener diode327. The negative voltage on conductor 325 increases to regulate theconduction of transistor 308 until no more negative feedback currentflows through potentiometers 350 and 349. With time rate potentiometer349 regulated to zero time delay, the rotor-wound induction motor 301performs with instantaneous current regulation and may be switched by amotor controller (not illustrated) to a direct reversing of the rotorspeed.

FIG. 4 is a fragmentary view of a modified version of an electronicrotor control system. Since the control circuits associated with thethree phases are identical, only one phase is illustrated and described.It is assumed that an overload control and starting control similar tothat described in FIG. 3 functions as a control means for transistor 408in FIG. 4 as well as for transistor 508 in FIG. 5, and is thereforeomitted. A variable inductive means is shown to serve as a phaseshifting means for controlling the conductance of thyristor 416 which isin series with one of the power conductors to the motor.

As the stator of the aforementioned induction motor (not illustrated) isenergized, a voltage is induced in the secondary rotor circuit. Thevoltage appears in rotor conductors 421 and 418, and since transistor408 at that instant represents an open circuit, the primary windings oftransformer 415 represent a maximum impedance. Through voltage andtransient limiting resistor 466 an AC voltage is conducted across theparallel resonating tank comprising inductor 470 and capacitor 468. Thetank circuit represents a maximum impedance to the sixty hertz input ofvoltage. However, the resonating circuit represents a low impedance torelatively high frequency noise generated by inductive switching, andserves to conduct the inductive spikes to conductor 421. To furthereliminate higher voltage transients, a bidirectional breakdown diode 457is employed to regulate the higher voltage transients to the AC voltagelevel. The maximum phase shifted voltage through resistor 461 chargescapacitor 417 to trigger thyristor 416 through resistor 460 into lowconduction. The purpose of resistor 460, as well as resistor 560 in FIG.5, is to reduce static charging of capacitors 417 and 517 by wayinternal capacitance of thyristors 416 or 516. The low current induces avoltage in the aforementioned current transformer and starting networkas shown in FIG. 3. A positive voltage rises on conductor 412 toincrease the conduction of transistor 408, and the increased flow of ACcurrent through the secondary windings of transformer 415 reflects onthe primary winding to lower the impedance of primary windings oftransformer 415. Reduction of the impedance of the primary windingsresults in a decreased phase shifted voltage charging capacitor 417 andan increased conduction through thyristor 416.

An external speed control is provided to regulate the rotor speed by thefunction of slip ring 452 on the rotor shaft. Since potentiometer 454 isconnected to ground, only one slip ring is needed where conductor 453 isconnected to speed controlling potentiometer 454 having a variableresistance against ground potential.

For an emergency operation, or if only one rotor speed is desired,potentiometers 454 and 466 may be replaced with a jumper 440, or a fixedvalue resistor. One more means is provided for an emergency operation ofthe motor in case the electronic starting control or current controlfails. A jumper 447 connected across the collector-emitter region oftransistor 408 will bypass the transistor. The motor can be started inlow speed, and controlled manually with potentiometer 454.

A constant speed feature is provided in the system of FIG. 4.Potentiometer 466 is electrically connected to the centrifugallyoperated plunger 433 which is connected through the sliding contact 452to ground potential. The constant speed regulating potentiometer 466 isfastened to the epoxy mold containing the electronic rotor controlcircuit which is mounted on the rotor shaft. The constant speedregulator shown in FIG. 4 is adjustable by the function of the fixedadjustment screw 446 in the elongated slot 448, and speed regulatingspring 430 and adjustment plate 432 pivots on the fixed fastening screw449, and pointer 444 shows the rated speed on dial 445. As the rotatingrotor develops speed, plunger 433 is forced upward until the force onretaining spring 430 is equal to the centrifugal force. This functionincreases the resistance through potentiometer 466 resulting in adecreases speed which will remain at a constant rate.

FIG. 5 represents a partial view of an electromagnetic control versionof a wound rotor control circuit. Instead of the function of transformer415 in FIG. 4, variable inductor 515 is incorporated to function as aseries resonance circuit with capacitor 558, and tuned for maximumresonance when ferrous core 534 is centered in inductor 515. Assumingthat the stator of the rotor wound motor controlled by FIG. 5 (notillustrated) is energized, an AC voltage is induced in the secondaryrotor circuit represented by conductors 521 and 518 in FIG. 5. Theresulting maximum phase shifted voltage by the maximum impedance fromthe unturned inductor 515 triggers thyristor 516 at a minimum rate ofconductance. As the aforementioned positive control voltage rises at apredetermined rate, transistor 508 is energized to conduct a DC currentthrough a high impedance solenoid 572 from the regulated positivevoltage through resistor 575, and across voltage regulating zener diode574 to the negative grounded conductor 531. The reverse polarized diode583 across transistor 508 serves to protect transistor 508 frominductive voltage breakdown. As the rising current increases, themagnetic force in solenoid 572, the north pole of permanent magnet 578facing the north pole of the rising magnetic force in solenoid 572,creates a repelling motion while ferrous core 571 is attracted towardsolenoid 572. The corresponding movement of control rod 533 causesferrous core 534 to move from position C toward position D. Thisfunction causes inductor 515 to approach resonance with capacitor 558,and the resulting lowered impedance functions to reduce the phase shiftof the voltage through resistors 566 and 561 charging capacitor 517, andan increased conduction occurs through thyristor 516. Damage tothyristor 516 is prevented by bidirectional breakdown diode 556. Theresulting increased speed generates a centrifugal force moving controlrod 533 toward constant speed regulating solenoid 573. However, aspermanent magnet 578 approaches the magnetized solenoid 573 having equalpolarity, the magnetic force will counter-balance the centrifugal forceacting on control rod 533. The aforementioned magnetic cores tend toassume a position predetermined by the amount of magnetic force insolenoid 573 which is regulated through slip ring 552 and a constantspeed regulating potentiometer 554 regulating the DC current from thepositive voltage on conductor 581 in respect to the grounded negativeconductor 580. As the aforementioned stator is de-energized, transistor508 opens to de-energize solenoid 572 and control rod 533 returns toposition C of ferrous core 534 by the magnetic force of solenoid 573.

FIG. 6 is a sine wave schematic diagram showing the phaseshift methodused in operating the aforementioned motor current controllingthyristors of FIGS. 1, 2, 3 and 9.

The voltage wave forms on sine wave center lines A, B and C,representing the corresponding phases of the FIG. 1 motor controlcircuit. The negative voltage of waveform reference lines D, E and F,represents the thyristor control voltage charging capacitors 117 of FIG.1, and have a variable phase relationship in respect to the three phasevoltage of sine wave reference lines A, B and C. One positive and onenegative half wave is illustrated at each of the three phase anglesshown, with the opposite potential of each respective phase beingsuperimposed with phantom lines on the opposite side of each illustratedconducting wave form as a means for easier understanding of itsfunction.

As the positive half cycle 601 appears on phase A, the negative triggercycle 626 on reference line D rises to reach point 625 representing thetrigger level of a thyristor similar to thyristor 116 of FIG. 1, andfunctions to cause such thyristor to conduct the remaining portion untilthe positive waveform reaches zero value, which functions to turn thethyristor off. However, as the positive portion 624 is conducting, onlya small portion of the conducting area 624 is rated as an effectivevoltage value, since the effective voltage is predetermined by theinstantaneous negative value of the respective phases B and C.

As can be observed by the dotted line 637, only a small portion of thenegative sine wave 614 is within the conducting phase angle of theconducting positive waveform 624, as shown by the shaded effectivevoltage waveform 615, which indicates the effective positive voltage, aswell as the effective negative voltage shown as part of the superimposeddotted lines representing negative waveform 614 of phase B. The negativewaveform 614C of phase C is also shown superimposed with a small portionof its conducting area 616 in phase with conducting positive region 624.

The next following cycle is the negative half cycle which is fired bythe second negative waveform of control phase F, followed by thepositive half wave 601B of phase B. As can be observed, a three phaserotating function is obtained with a low effective voltage.

As the phaseshift of the negative firing voltages of phases D, E and Fis decreased from 95 to 80 degrees, a large increase occurs to theamplitude of the effective voltage as can be observed between the heavydotted lines 635 and 636. However, the process of effective voltagedistribution is the same as previously described.

The change of the trigger voltage in phase relationship to the threephase waveforms A, B and C can be observed at points 631 and 632, andappears in each of the negative waveforms D, E and F. As theaforementioned phase relationship is reduced to 50 degrees, almost 90percent of each sine wave is conducting as effective voltage.

An added function appears at this increased rate of conduction. Inreferring to the conduction of positive waveform 609 of phase A, thenegative potential of waveforms 608B and 608C from phases B and C areshown superimposed with dotted lines, and the effective voltage is shownas shaded area. The negative curve 608B is shown overlapping thenegative curve 608C in the cross shaded area 619. Since area 619contains the voltage distributed to the negative potential of phase B,as well as phase C, the sum of the two voltages is equal to the amountadded in portion 621, which is in proper relationship to the three phasesystem.

FIG. 7 is a schematic showing a modified version of the FIG. 1 overloadcontrol designed to control the overload function of a DC motor. Insteadof current transformers 102 in FIG. 1, resistor 702 is provided toproduce a low voltage drop for sensing the DC motor current with a morepositive polarity on conductor 721 in respect to the negative voltage onconductor 731. The electronic circuit controlling transistor 52 in FIG.1 is modified to be controlled by motor conductors 721 and 722 in FIG.7. As the DC motor 701 is started by depressing starter button 711,contactor coil 715 is energized, closing holding contact 716 and motorstarting contacts 717 and 718. The DC voltage on conductors 719 and 720appears on conductors 722 and 731; the voltage on conductor 731 isconnected through resistor 702 to conductor 721. As the negative voltagecharges capacitor 756, the positive voltage on conductor 722 throughmotor 701 and resistor 757 charges the capacitor 756 on conductor 758 toan equal voltage value. The consequent momentary positive voltagethrough resistor 751 is regulated through zener diode 754. Through basecurrent limiting resistor 753, transistor 752 is thus turned off tointroduce variable resistor 704 into the overload control circuit at aset resistance value predetermined by the normal starting current ofmotor 701. The charged capacitor 756 discharges across variable resistor755 at a time rate predetermined by the set resistance value thereof,which is established in accordance with the normal starting time ofmotor 701. Variable resistor 755 is rated at a lower value than resistor757, and as capacitor 756 is discharged, a negative voltage appears onconductor 758. Through resistors 751 and 753, transistor 752 issaturated by bypass resistor 704, and the overload control will gain asensitivity dictated by variable resistor 705 which is set according tothe normal load current of the DC motor. The voltage through resistor705 is regulated through diodes 747 and resistor 748, the combination ofwhich serves as a nonlinear resistor. As a tripping-device to activatetrip relay 707, a silicon controlled rectifier 751' is employed. Sincethe gate tripping voltage is of a very low value, diodes 747 serve thefunction of zener diode 47 in FIG. 1. A negative bias for a more stableturn off operation is provided through resistor 726 in series withmicroammeter 727. The microammeter functions as a useful motor currentmonitor.

As the voltage charging capacitor 706 reaches the rated "turn on"voltage of SCR 751', the self-latching SCR fires, and the negativevoltage on conductor 731 appears on conductor 758'. The resultingcurrent will illuminate trip indicator light 714, and energize triprelay 707 to open the normally closed relay contact 709 de-energizingthe holding circuit for contactor relay 715. To reset the latched SCR751', reset button 710 is depressed momentarily, and the SCR 751' isrestored to the normally de-energized state. With trip relay contact 709closed, motor 701 can be started again. The motor starting circuitcontrolling contactor relay 715 is energized from motor supplyconductors 719 and 720 through fuses 742 and 743. As motor startingbutton 711 is depressed, the negative voltage on conductor 737 feedingthrough normally closed trip relay contact 709, conductor 733, normallyclosed stop-button 712 and conductor 734, energizes the coil of motorstarting contactor 715 which is fed positive voltage through conductor736. As contactor relay contacts 717 and 718 pull in to start motor 701,contact 716 pulls in to bypass the momentary start button 711, servingto latch contactor relay 715.

An electronic discharge circuit similar to the overload circuit in FIG.1 is provided. As capacitor 706 is charged by the voltage drop acrossresistor 702, the positive voltage on conductor 762 is always morepositive in respect to the positive voltage on conductor 729, and servesto keep the transistor 768 turned off. Diodes 765 function to protectthe base emitter region of transistor 768 from a reverse voltagebreakdown. As a higher voltage charges capacitor 706 close to the ratedtrip value of SCR 751' before decreasing to a low value, the voltage onconductor 762 decreases, and the negative voltage on conductor 731through resistor 767 turns transistor 768 on to discharge capacitor 706until the stored voltage on the capacitor is negative in respect to thevoltage on conductor 762. The more positive voltage from conductor 762through resistor 769 will again turn off transistor 768 permittingcapacitor 706 to be charged through variable resistor 705.

FIG. 8 represents a simplified electro-magnetic version of the overloadcontrol designed to control the load current feeding a single phase ACmotor.

Contact 811B of motor start button 811A serves to introduce variableresistor 804 into the overload control circuit as motor 801 is beingstarted, and functions similar to the electronic circuit controllingtransistor 752 in FIG. 7, as a time delay provided by resistor 755 inFIG. 7. Push button 811 must be depressed for a time required by motor801 to reach a near full speed. Trip relay 807 is provided to serve thefunction of silicon controlled rectifier 751' in FIG. 7.

To afford a latching function in trip relay 807, a positive voltage isprovided on conductor 830 in respect to the common negative conductor831. The voltage rises on conductors 825 and 862 from an excessive motorcurrent through motor 801 feed conductors transmitted through currenttransformer 802, and the high voltage exceeds the rated time delay ofresistor 805 and capacitor 806. The voltage between conductors 829 and831 rises to energize trip relay 807, and as relay contact 809B opens tode-energize motor contactor relay 815, relay contact 809A closes toilluminate trip indicator light 814. The positive voltage throughblocking diode 813 functions to retain trip relay 807 in a latched stateuntil reset by depressing of the normally closed push button 810. Duringnormal operation the blocking diode 813 prevents the positive voltage onconductor 829 from flowing to the negative conductor 831 through tripindicator light 814.

A thyristor controlling means similar to FIG. 3 is shown in FIG. 9controlling the function of a DC motor. Armature 901 is shown fed by athree phase thyristor controlled circuit similar to as shown in theaforementioned control circuit FIG. 1. Area 921 shown in phantom linesrepresents the control circuit and variable transformer 115 of FIG. 1,and said transformer may be provided to function with the inductivecontrol means as shown controlling transformer 915 in FIG. 9. Anoverload control circuit similar to FIG. 1 operated by currenttransformers 102 in FIG. 1 is represented by the dotted line area 922,and is operated by current transformer 902. The three phase voltageproduced in conductors 921B by thyristors 916B is rectified through fullwave rectifiers 923 to energize DC armature 901. The AC ripple of therectified DC current in conductors 939 is induced in current transformer902 which is rectified to feed back through the aforementioned describedmeans.

A circuit similar to that feeding DC motor armature 901 feeds DC meansshunt winding 927. A DC polarity reversing means is provided forreversing the direction of rotation of DC motor armature 901 by thefunction of the double-pole double-throw relay contacts 926 operated byrelay 929 which can be operated by forward and reverse speedpotentiometer 905, when the DC current through shunt winding 927 is atmaximum rate only, which corresponds to the lowest speed of DC armature901. To increase the speed in the forward direction, pointer 906 ofpotentiometer 905 is turned counter-clockwise, decreasing the resistanceto increase the magnetic field of solenoid 912. The resulting functionattracts ferrous core 978 to lift control rod 933 upward, serving todecrease the magnetic conduction through transformer 915, and toincrease the phase shift of the control voltage regulating thyristors916A to a lower rate of conduction. The resulting lower current throughshunt winding 927 increases the speed of the DC motor armature 901. Toreverse direction of motor rotation, the pointer 906 of potentiometer905 is turned clockwise, decreasing the motor speed. As potentiometer905 is centered, minimum motor speed is obtained with maximum current inshunt winding 927. As pointer of potentiometer 905 is turned furtherclockwise, switch 930 is closed by the pointer arm 906 and relay 929activates by the double-pole double-throw switch 926 serving to reversethe polarity of the DC voltage feeding shunt winding 927.

FIG. 10 shows bidirectional thyristor 1008 controlling the function ofelectro-magnetic contactor relay 1015 in a similar manner to the controlcircuit shown in FIG. 7. The control circuit shown operating thebidirectional thyristor 1008 is a manufacturer's circuit which ismodified to be overload tripped by a circuit similar to FIG. 7 wheresilicon controlled rectifier 751 is represented as the SCR 1051 in FIG.10. Instead of Dc motor load resistor 702 in FIG. 7, a single phasecurrent transformer and the associated circuitry of FIG. 1 is assumed tobe applied to the function of FIG. 10 overload control.

As motor contactor relay 1015 is energized by depressing push button1011, thyristor 1008 switches on with a self-latching function providedby resistor-capacitor circuit 1017 to energize contactor relay 1015. Theoverload trip means is accomplished by the function of SCR 1051 whichserves to short the DC voltage rectified by full wave rectifiers 1003when SCR 1051 is tripped by the motor overload control chargingcapacitor 1006. A self-resetting overload trip function is achieved inthis manner. However, if a latching function is desired, capacitor 1033in series with resistor 1034 to prevent instantaneous discharge isemployed to reduce the AC ripple resulting in a latching function of SCR1051. Jumper 1032 is then removed, permitting push button 1010 tofunction as an overload trip resetting means.

FIG. 11 is a circuit showing a thyristor representing a gate controlledswitch or GCS controlling the function of a single phase AC-DC motor, ina similar manner as bidirectional thyristor 1008 controllingelectro-magnetic contactor relay 1015 in FIG. 10.

An AC or DC voltage can be applied to conductors 1114 and 1116, andduring AC operation, a full cycle performance is obtained by thefunction of full wave rectifiers 1103 which, being in series with GCS1151, functions to provide a higher voltage breakdown performance thanthyristor 1008. Full wave rectifiers 1103 in series with GCS 1151 alsofunction to block the high inductive reverse voltage which occurs duringthe fast switching of the inductive load of motor 1101.

To prevent a voltage feedback through thyristor diode 1130 when astarter button 1111 is depressed, a regulated voltage is obtainedthrough resistor 1122 across zener diode 1121 and capacitor 1109,serving to provide a low gate trigger voltage and sufficient gatecurrent.

As push button 1111 is depressed, the positive voltage through resistor1104 turns on gate controlled switch 1151, having a latching function asa result of ripple reducing capacitor 1133, which serves to maintain aminimum holding current, by the function of the delayed discharged ofcapacitor 1133 through resistor 1134. The circuit comprising capacitor1133, resistor 1134 and diode 1105 also represents the manufacturer'srecommended circuit for providing a capacitor shunt to increase gateturn off ability.

An electronic overload control similar to that used in the FIG. 1 motorcontroller is assumed to be applied through the function of a singlephase current transformer applied to conductor 1116 or 1114.

Assuming that motor 1101 approaches an overloaded condition, capacitor1106, which corresponds in function to capacitor 6 in FIG. 1, is chargedto the switching level of thyristor diode 1130. The gate controlledswitch 1151 is a thyristor which is different from the aforementionedsilicon controlled rectifier in that the GCS 1151 can be turned off witha predetermined value of negative gate voltage and current.

As thyristor diode 1130 switches on, the stored voltage in capactior1106 is discharged through gate current limiting resistor 1104 to turnoff GCS 1151.

FIG. 12 represents an electronic motor braking means controlling aselective transformer similar to that shown in FIG. 2. A magneticoperating means is shown controlling ferrous cores 1234, and spring 1222is provided to retain the ferrous cores in the center off position whenmagnetic control solenoids 1201 and 1202 are de-energized. Air gaps 1209are provided as an equal distribution means of the magnetic lines offorce between phase A and phase C on the secondary side of transformer1215. Air gap 1209 represents a magnetic resistance similar to air gap1210 when cores 1234 are in the position shown in FIG. 12. When the peakmagnetic flux occurs between phase A and phase C, ferrous cores 1234 aremoved upward by an increased repelling force caused by an increasingmagnetic force in solenoid 1202 that is polarized to repel permanentmagnet 1228. Meanwhile, the magnetic flux through solenoid 1201decreases in opposition to the equal polarity faced by permanent magnet1228. A resulting firm upward motion functions to place ferrous core1234B across the secondary windings 1255A of phase A. The magnetic linesof force through the secondary side of transformer 1215 encounter theresistor of air gap 1209 which functions to partially divert magneticflux across the parallel path of ferrous core 1234B. This has acancelling effect to the induced voltage in secondary windings 1255A, asdescribed in the FIG. 2 detailed description.

An electronic braking system of the aforementioned electric motor isaccomplished by the function of permanent magnets 1251 shown fastened tothe rotor shaft. The permanent magnets 1251 are mounted with the samemagnetic polarity facing induction coil 1250, and as the magnetic polesrotate with the rotor shaft, a pulsating DC voltage is generated inelectromagnetic induction in coil 1250.

Since the polarity of the induced voltage is predetermined by thedirection of movement of the permanent magnets across induction coil1250, the forward and reverse directions of the rotor are represented byrespective positive and negative voltages.

As the aforementioned motor is started forward, push button 1239 withganged auxiliary contact 1235 is depressed. A positive voltage fromconductor 1242 through relay coil 1227 is regulated across voltagereference diode 1232 through resistor 1253, and by the function ofcapacitor 1252. The lowermost regulated voltage provides sufficientcurrent to trigger gate controlled switch 1251 through the depressedstart button 1239. As GCS 1251 latches, the negative voltage onconductor 1241 through stop switch 1237 energizes relay coil 1227 andcauses relay contacts 1203, 1224 and 1223 to pull in. Since auxiliarypush button switch 1235 also is depressed, the positive voltage fromconductor 1242, through base current limiting resistor 1226, turns onforward speed transistor 1211. Consequently, the positive voltagethrough current limiting resistor 1214 produces a positive voltage onconductor 1257, the magnitude of which is predetermined by the set valueof potentiometer 1205. Transistors 1218 and 1219 are each forward biasedthrough resistors 1254 and 1256, respectively, to generate an equalmagnetic force in each of the solenoids 1201 and 1202.

As the positive voltage on conductor 1257 feeds through base currentlimiting resistors 1216 and 1217, the positive voltage increases theconducting through forward speed transistor 1219 while reversetransistor 1218, being a PNP transistor, decreases its conduction.

The increased current through solenoid 1202 creates a magnetic forcewith its south pole facing the south pole of permanent magnet 1228. Theequal polarity functions to create an upward motion, and as the currentthrough solenoid 1201 decreases, a lesser force resists upward movementof permanent magnet 1228. As a result, a firm and accurate control ofselective transformer 1215 is achieved by controlling the voltage levelon conductor 1257.

As soon as the aforementioned motor as in FIG. 1 has obtained rotatingmotion, permanent magnets 1251 induce a DC voltage in induction coil1250 that is more positive in respect to negative conductor 1241, andthrough resistor 1249, the voltage is regulated across a bidirectionalvoltage reference diode 1248 to a specific voltage level. Such specificvoltage operates forward speed linear amplifier 1207 through negativevoltage blocking diode 1246. A resulting positive voltage is produced bythe amplifier, and since relay control 1223 is closed by relay-coil1227, the positive voltage feeds through contact 1223 to energizeforward control transistor 1211, providing a positive voltage throughresistor 1215 for forward operation of transistor 1219.

The motor can be stopped by depressing the normally closed stop button1237 to unlatch gate controlled switch 1251, or a turn off may occur bythe function of an overlaod control as provided in FIG. 1, to provide aturn off function similar to as described in the FIG. 11 turn offcircuit.

As relay coil 1227 is de-energized, relay contact 1203 opens todisconnect the speed-controlling voltage from potentiometer 1205, andrelay contacts 1223 and 1224 are switched to the de-energized positionillustrated in FIG. 12. As can be observed, a reversing function isaccomplished by switching the positive output voltage from forwardamplifier 1207 to energize reverse control transistor 1212, therebyswitching negative voltage through current limiting resistor 1213 tosaturate reverse transistor 1218 for instantaneous reverse operation ofselective transformer 1215.

The reversing time required is predetermined by the adjusted value ofthe instantaneous current control means similar to as described in theFIG. 1 detailed descriptions. At the instant the motor approaches zerospeed, permanent magnets 1251 no longer induce voltage in induction coil1250, and the motor comes to full stop. If attempts is made to turn therotor shaft while relay coil 1227 is de-energized, whether in a forwardor reverse direction, the movement is opposed by a counter-movement witha speed approximately equal to attempted movement of the rotor shaft,since the voltage induced in induction coil 1250 is linearlyproportional to the rate of speed, and the polarity of the inducedvoltage predetermined by the direction of armature rotation.

Amplifiers 1207 and 1208 are linearly proportional to the input voltage,and are switched inverted in respect to control transistors 1211 and1212 while relay coil 1227 is de-energized, serving as an opposing meansto the movement of the rotor shaft.

Bidirectional breakdown diodes 1220 and 1221 serve as inductive voltagesurge protectors for transistors 1218 and 1219, respectively.

The reversing capability of the FIG. 12 circuit can be adapted to thenon-mechanical transformer flux control circuit of FIG. 1. Forwardtransistor 1219 and reverse transistor 1218 are controlled as describedabove in conjunction with FIG. 12. The outputs of the forward andreverse transistors are connected to control the phasing of the flux inthe transformer corresponding to transformer 115 in FIG. 1. Forwardtransistor 1219 is connected to drive first or second secondary windingsand reverse transistor 1218 is connected to drive the first secondarywinding and a third secondary winding. The second secondary winding isassociated with the phase that leads the phase associated with the firstsecondary winding, and the third secondary winding is associated withthe phase that lags the phase associated with the first winding. Theremaining functions, e.g., starting and overload protection, areachieved through transistor 108, which is controlled as described inconjunction with FIG. 1. Transistor 1218 and 1219 each control thecurrent in two secondary windings, whereas transistor 108 controlscurrent flow in all secondary windings.

While I have shown and described specific embodiments of my invention, Iam aware that many modifications may be made without departin from thespirit of the invention. To mention as a typical example, I like torefer to the motor overload controls of FIGS. 7 and 8. The motor currentsensing resistor 702 of FIG. 7 may readily be applied to the currentcontrol of an AC motor, and the AC voltage drop across the said resistorrectified in similar manner as current-transformer 802 in FIG. 8.Therefore, I do not intend to limit my invention to the specificarrangements shown.

What is claimed is:
 1. A multi-level trip circuit for electrical loads having high unit current demands when they are connected to an electrical source for applying energy thereto comprising:electrical connection means for connecting a load to an electrical source, said electrical connection means including a controllable switch means for connecting and disconnecting said load to said electrical source; current sensing means associated with said electrical connection means operable to develop an electrical signal proportional to the instantaneous current passing through said electrical connection means; circuit means connected to receive said signal for said sensing means, said circuit means having at least a first resistance and a second resistance connected in series, arranged so said electrical signal passes serially therethrough; switching means connected across one of said first and second resistances operable to bypass one of said resistances when said switching means electrically conducts; a timing circuit connected to receive said electrical signal and connected to said switching means being operable to activate said switching means to bypass the resistance across which it is connected at a preselected time interval based on the level of said electrical signal; and a control means connected to the output of said circuit means and to said controllable switch means operable to cause said switch means to dissconnect said load from said electrical source when said electrical signal exceeds predetermined levels whereby a higher initial current can be drawn by said load for a preselected time interval without causing said switch means to disconnect the load and thereafter a lower current level will cause the switch means to disconnect the load after the preselected time interval has been exceeded.
 2. The multi-level trip circuit defined in claim 1 wherein a third resistance is connected in series with the first and second resistances, said third resistance being thermally responsive to heat generated by the load to reduce its resistance, whereby the trip circuit will trip at a lower current as the temperature of the load increases.
 3. The multi-level trip circuit defined in claim 1 wherein the timing circuit contains a capacitor. 